To simplify the following discussion, the present invention will be discussed in terms of a memory based on non-destructively read ferroelectric memory cells. These cells may be viewed as a field effect transistor (FET) in which the gate oxide has been replaced by a ferroelectric layer. The ferroelectric layer may be polarized in one of two directions. The polarization exerts an electric field on a semiconductor layer whose resistance is altered in response to the direction of polarization. Conversely, the direction of polarization, and hence, the value of the stored data bit, can be determined by measuring the resistance of the semiconductor layer. The resistance is normally measured by observing the current that flows between two electrodes that are spaced apart from one another and in contact with the semiconductor layer. These electrodes are analogous to the source and drain terminals on a conventional FET.
The direction of polarization of the ferroelectric layer is set by manipulating the voltages on the source and drain electrode together with a third "gate" electrode which is disposed on the opposite side to the ferroelectric layer from the semiconducting layer. Hence, care must be taken during read operations to assure that the voltage applied to the electrodes is not sufficient to disturb the contents of the memory cell. If the read voltage is substantially less than the coercive voltage of the ferroelectric layer, the contents of the memory cell will not be lost during the read operation.
Unfortunately, limitations on the read voltage lead to reduced reading speed when conventional sense amplifiers are utilized. The time needed to sense the current is determined by the rise time of the bit lines and the sense voltage. The sense amplifier requires a voltage excursion of some minimum value to distinguish a "1" from a "0". The time required for this voltage swing can be reduced if the voltages applied to the circuit are increased.
In addition, conventional sense amplifiers of the type used to read EEPROM memory cells have difficulty providing the low voltage levels needed to read ferroelectric FETs. The read voltages supplied by these amplifiers are determined by the threshold voltage of a gate within the amplifier. Unfortunately, the typical threshold voltages are too high to provide a regulated source of read voltage at the level required to prevent the contents of the memory cell from being disturbed.
Broadly, it is the object of the present invention to provide an improved sense amplifier.
It is a further object of the present invention to provide a sense amplifier with a reduced settling time.
It is a still further object of the present invention to provide a sense amplifier that can generate lower read voltages than prior art sense amplifiers.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.